In order to achieve a high level of automated operation in a plant, a factory, or the like, a distributed control system (DSC) has been implemented, in which on-site devices (measures and actuators), which are referred to as field devices are connected via a communication means to controllers that controls the field devices. Although the communication system that forms the base of such a distributed control system had been almost always one that communicates by cable, ones that communicate wirelessly in conformance with an industrial wireless communication standard such as ISA100.11a or WirelessHART (registered trademark) have been realized in recent years.
The wireless communication system in conformance with these wireless communication standards includes a management apparatus, which is referred to as a system manager (or a network manager), and manages communication resources (channels, time slots, and the like) required to communicate wirelessly via wireless networks. In particular, the management apparatus creates a communication schedule in which different time slots and channels are assigned to wireless communications from each other, which are performed via the wireless network, thereby, manages the wireless communications so as to prevent the overlap of the assignments of the communication resources.
The above-stated distributed control system regularly collects the measured results of field devices and regularly controls (operates) the field devices based on the collected measured results. Therefore, the above-stated management apparatus of the wireless communication system creates a periodical communication schedule by using a communication template, which is referred to as a super frame as shown in FIG. 14 and setting communication links between the wireless devices in the super frame (shadow regions in FIG. 14). FIG. 14 is a diagram for describing the super frame.
The above-stated communication link consists of the following information (1) to (4).
(1) Information specifying the time slot at which the transmittance and reception of the wireless signals is performed.
(2) Information specifying the channel used for the transmittance and reception of the wireless signals.
(3) Information specifying the transmittance or the reception.
(4) Information specifying the super frame to which the communication link belongs.
The information represented by the above-stated (1) consists of information (offset) indicating the amount of deviation from the time slot at the front of the super frame and information (interval) indicating the interval between the time slots during the periodic communication in the super frame.
Each of the following patent references 1 to 3 discloses an example of the above-stated conventional wireless communication system. The following non-patent reference 1 discloses a method for managing communication resources in conformance with the above-stated ISA100.11a.
In the above-stated wireless communication system, for example, when a new wireless device is taken part in a wireless network, the communication link is dynamically set by a management apparatus. When such a new communication link is set, the overlap of the new communication link with the previously set communication link is required to be prevented, but, conventionally, there are some cases where a new communication link cannot be set even if a plurality of empty slots exists and the communication resources are not effectively used.
FIG. 15 is a diagram showing an example of communication links to be set in the super frame. The following four communication links “A” to “D” are set in the example shown in FIG. 15. In FIG. 15, time slots, to which the characters “A” to “D” are assigned, each represent time slots, to which the following communication links “A” to “D” are set, and time slots, to which the characters “A” to “D” are not assigned, represent empty slots. In order to simplify the description, FIG. 15 shows only one assignable channel.
Communication link “A”: offset=0, interval=8
Communication link “B”: offset=1, interval=4
Communication link “C”: offset=3, interval=16
Communication link “D”: offset=6, interval=12
When a new communication link having an interval “4” is tried to be set under the condition that the above-stated communication links “A” to “D” have been set, it is found that the new communication link cannot be set even if empty slots exist as shown in FIG. 15. As described above, the case in which a new communication link cannot be set even if empty slots exist is referred to as a “fragmentation” in this description and the like.
The management apparatus in the wireless communication system frequently performs an operation for deleting the previously set communication link and setting a new communication link (resetting a communication link) by the exchange of path, the exchange of bandwidth, and the like. If such a communication link resetting is frequently performed, this progresses the fragmentation of the communication link and increases the probability that the setting of a new communication link will become impossible, and it is likely that the use efficiency of the valuable communication resources are further decreased.